This invention relates generally to still cameras, and more particularly to cameras utilizing 35 mm film or the like having a film threading tongue which extends from a cassette, and provided with motor driven film advancing means which, upon loading the camera with film, automatically prewinds the film onto a takeup spool and, after each exposure has been made, advances the film back into the film cassette one frame length at a time.
The 35 mm film format had its genesis in the early days of motion picture production, when still cameras were designed to make exposures on 35 mm movie film so that set lighting and camera exposure could be accurately checked. This format was substantially smaller in size than those commonly in use at that time, and as its popularity increased, a great number of cameras were designed to accommodate the 35 mm format.
Today, 35 mm photographic film is most commonly provided in pre-manufactured and pre-loaded film cassettes. The cassettes are light-tight and have a relatively short length of film or "leader", extending therefrom to enable the film to be engaged with a take-up spool rotatably mounted in a film-receiving chamber on the interior of the camera. This leader is fully exposed, and is not usable for the taking of pictures. Along each edge of the film, a series of regularly spaced sprocket holes are formed, reminiscent of the days when the film was intended to be run through a motion picture camera. Today, the sprocket holes are used to engage portions of the internal working mechanism of the camera in order to advance the film from the cassette, as needed. Commonly, a first portion of the leader extending from the cassette is trimmed such that a narrowed "tongue" is formed. It is this tongue that is engaged to the take-up spool within the camera.
In a typical 35 mm format camera, a film advance lever is connected by a clutch mechanism to the take-up spool. Each time the film advance lever is operated, the shutter is cocked and a take-up spool is rotated and a fresh, unexposed portion, or "frame", of film is brought into position at the imaging area of the camera, i.e., that portion of the camera to which light is directed through the lens when the camera's shutter mechanism is operated. The exposed film is thus progressively wound onto the camera's take-up spool.
When the supply of film has been exhausted, the take-up spool clutch is disengaged, and a rewind lever is used to rotate the spindle within the film cassette in order to draw the exposed film back into the cassette where it will be protected from incident light. Thereafter, the camera may be opened and the film cassette removed for processing.
It is well known in the photographic art to provide motorized film transport systems which automatically advance the film after each successive exposure has been made. This does away with the necessity to operate a film advance lever after each picture-taking operation, but still requires that the film be rewound into the cassette once the roll of film has been completed. Some such cameras utilize a motorized rewind system where the film is wound back into the cassette once the roll has been completed. This, however, does not provide protection for the film in the event that the camera is opened before the exposed film has been wound back into the cassette.
It is also known within the photographic art to provide cameras with mechanisms enabling the film within a film cassette to be completely "prewound" onto the camera's take-up spindle prior to the taking of exposures. Thereafter, when an exposure is taken, the film is advanced toward the cassette, i.e., rewound into the cassette. In this manner, exposed frames are protected within the cassette. Film frame counters in such cameras driven by film movement in both pre-wind and rewind directions indicate on a counter dial the remaining unexposed frames on the film. Should the camera become accidently or inadvertently opened, the incident light would thus not reach the exposed frames already wound into the cassette, but only the unexposed film segments left on the take-up spool, and any exposed frames or portions of frames found intermediate the imaging area and the cassette.
Another problem created when film is advanced into the cassette rather than from the cassette stems from the fact that the leader has already been exposed, and any attempt to use this portion of the film to take a photograph will be unsuccessful. We are unaware that others have attended to or considered this problem. Therefore, it is desirable that no portion of the film leader is inadvertently positioned within the imaging area during a picture taking operation, giving the photographer who forgets to observe the film counter dial the impression that there is an unexposed film segment in imaging position when, in fact, the film segment has already been wholly or partially exposed and thus incapable of recording a complete image. Such a system must also take into account the fact that 35 mm film is typically sold in varied lengths, and, more importantly, in lengths which are not integral numbers of frame lengths and so a count down film counter dial can stop at a point giving a confusing between frame count, so that the user will not know whether he has one or no frames left for picture taking.
Such a system must also take into account the fact that film cassettes are available which may be bulk loaded by the photographer, which means that the photographer may himself load unexposed film into individual cassettes. Such loading techniques may provide for more or fewer exposures than typically commercially available in pre-loaded cassettes. Commercially pre-loaded cassettes are typically available in 12, 20, 24, and 36 exposure cassettes, and the photographer who loads his own cassettes may include any number of exposures up to the maximum length of film which may be accommodated by the film cassette.